The ability of direct injection of non-replicating plasmid DNA coding for viral proteins to elicit protective immune responses in laboratory and preclinical models has created increasing interest in DNA immunisation. A useful review of DNA vaccination is provided in Donnelly et al, Journal of Immunological Methods 176 (1994) 145-152, the disclosure of which is incorporated herein by reference.
Intramuscular injection of DNA as a means of vaccination can induce both cellular and humoral responses (1). Studies using reporter proteins demonstrated that muscle cells are the principal target for transfection after intramuscular DNA injection (2). The mechanisms underlying the induction of immune responses after DNA immunisation are unclear. Since myocytes express MHC Class I at low levels and do not constitutively express Class II or costimulatory molecules such as B-7 (3), they appear unlikely candidates for the induction of Ab or CTL responses. It is possible that low level transfection of antigen presenting cells (APCs) occurs at the injection site and these APCs then traffic to lymphoid organs and present the encoded antigen to B and T cells (4) as has been shown after intradermal (5) and biolistic DNA immunisation (6). Alternatively the myocyte may act merely as a source of antigen and priming occurs in the draining lymph node. In the latter case, optimum immune induction would result if the antigen was released from the myocyte by secretion or subsequent to cell damage.
One strategy that has been shown to augment the response to polynucleotide, or DNA, vaccination is the use of sequences encoding cytokines or co-stimulatory molecules (Conry et al, (1996) Gene Therapy 3: 67-74). These investigators showed an increased response when the DNA administered encoded not only the antigen of interest but also for B7-1.
The present inventors investigated the effects of modifying the antigen such that it will be targeted to APC or sites of immune induction. This was shown to not only markedly enhance the immune response but also cause immune deviation.